A generic device has at least two supply circuits for providing the power/voltage supply and a protective device for intrinsically safe power limiting of the current supplied by the supply circuits to the consumer in the explosion-protected area, wherein the supply circuits as part of the protective device each have devices for controlling and/or limiting current.
Such devices are known and are used in installations in which parts of the operating means are operated in the explosion-protected area. The supply devices may be integrated in input/output components and are considered to be a link between an external arithmetic unit of a programmable logic controller and sensors and/or actuators in the explosion-protected area.
Operating means in plant sections which have been declared to be explosion-protected areas must meet particular requirements which are set out in European Standard EN 500 20. One option mentioned in EN 500 20 for operating means in the area at risk of explosion is so-called “intrinsic safety”. In this connection, the supplied energy is limited such that no incentive spark can occur. In order to limit the energy, both current and power must be limited. The options for realizing this specifically are likewise regulated by EN 500 20.
In relation to the area at risk of explosion, differentiation into so-called zones is usual in Europe. In this case, a distinction is made between zone 0, zone 1 and zone 2, wherein the most stringent requirements exist for zone 0 since a permanently explosive atmosphere is assumed in this case. In this area, only resistive limiting of the current is permitted. Therefore, only resistors are allowed but no electronic limiters for the current. That is to say that, in the case of an input/output component in which a current is intended to be conducted in the explosion-protected area in zone 0, said current must be limited by one or more resistors, wherein the resistor or resistors must likewise meet particular preconditions.
The functionality or availability of input/output components can also significantly increase in the explosion-protected area if said input/output components are redundantly supplied. That is to say the circuit in the explosion-protected area (zone 0, 1 or 2), which is also referred to as field circuit, is connected to at least two supply circuits having the same function, wherein one supply circuit is active and the other supply circuit is on standby. If a fault occurs in the active device, it is possible to automatically change over to the second device without significant disturbances occurring in the process.
Additionally, in the case of a redundant supply such as this of input/output components in the explosion-protected area, the respective safety requirements must, of course, be met, for example the safety level of the “intrinsic safety”. The intrinsic safety can be maintained, for example, by a changeover being effected between the two supply circuits such that in each case only one supply circuit is connected to the explosion-protected area. In order that an erroneous parallel connection of the two supply circuits, in the case of which the “intrinsic safety” would no longer be ensured owing to the addition of the currents of the two modules, does not occur there is a safety requirement on the changeover device used in the case of this implementation.
As a result of this, additional components are required, which is associated with an increase in complexity in terms of construction and an increase in costs. The device for redundant power/voltage supply itself can be configured such that it may be used at least in zone 1 or zone 2.
A connection unit for electrically coupling redundant assemblies to external communications interfaces is known from German utility model DE 94 21 001 U1, which external communications interfaces consist of parallel-connected plug-on-type units which are designed to receive current-limiting switching means.
Furthermore, an arrangement for connecting field devices arranged decentrally and in the immediate vicinity of the process to a remote central device is known from DE 101 35 980 C1, which central device has a plug unit that determines the configuration and is suitable for redundant connection of input/output assemblies and can have a current-limiting resistor in the event of a parallel connection.
What is common to the two known devices is that, although the common current-limiting resistor limits the current in the field circuit to permissible values in the event of a redundant connection of input/output assemblies, in the event of a failure of one of the redundant assemblies the required supply current strength for operating the connected field device is no longer provided for all application cases, however.
Moreover, an arrangement for redundant voltage supply of two-wire measuring transducers is known from DE 198 14 097 C1, in the case of which the redundant measuring transducer supply units are decoupled from one another by means of diodes and in each case only one measuring transducer supply unit supplies the two-wire measuring transducer while the respective other measuring transducer supply unit is operated in a function monitoring mode. What is disadvantageous is the necessary changeover procedure, in the event of failure of the supplying measuring transducer supply unit, to the redundant measuring transducer supply unit.
Finally, in the case of redundant supply of the connected field device via a common singular current-limiting resistor, it should be noted that the loop current in the supply circuit, even in the case of redundant current supply, is to be limited to permissible values and that the power loss via the singular current-limiting resistor in the redundant current supply device in the permissible short-circuit case in the field-device-side conductor loop leads to an impermissible overloading of the resistive current limiter in the current supply device.